Methods and systems for detecting a capacitance using switched charge transfer techniques

ABSTRACT

Methods, systems and devices are described for detecting a measurable capacitance using charge transfer techniques. According to various embodiments, a charge transfer process is performed for two or more times. During the charge transfer process, a pre-determined voltage is applied to the measurable capacitance, and the measurable capacitance is then allowed to share charge with a filter capacitance through a passive impedance that remains coupled to both the measurable capacitance and to the filter capacitance throughout the charge transfer process. The value of the measurable capacitance can then be determined as a function of a representation of a charge on the filter capacitance and the number of times that the charge transfer process was performed. Such a detection scheme may be readily implemented using conventional components, and can be particularly useful in sensing the position of a finger, stylus or other object with respect to an input sensor.

PRIORITY DATA

This application claims priority of U.S. Provisional Patent ApplicationSer. Nos. 60/687,012; 60/687,166; 60/687,148; 60/687,167; 60/687,039;and 60/687,037, which were filed on Jun. 3, 2005 and Ser. No. 60/774,843which was filed on Feb. 16, 2006, and are incorporated herein byreference.

TECHNICAL FIELD

The present invention generally relates to capacitance sensing, and moreparticularly relates to devices, systems and methods capable ofdetecting a measurable capacitance using switched charge transfertechniques.

BACKGROUND

Capacitance sensors/sensing systems that respond to charge, current, orvoltage can be used to detect position or proximity (or motion, presenceor any similar information), and are commonly used as input devices forcomputers, personal digital assistants (PDAs), media players andrecorders, video game players, consumer electronics, cellular phones,payphones, point-of-sale terminals, automatic teller machines, kiosksand the like. Capacitive sensing techniques are used in applicationssuch as user input buttons, slide controls, scroll rings, scroll stripsand other types of inputs and controls. One type of capacitance sensorused in such applications is the button-type sensor, which can be usedto provide information about the proximity or presence of an input.Another type of capacitance sensor used in such applications is thetouchpad-type sensor, which can be used to provide information about aninput such as the position, motion, and/or similar information along oneaxis (1-D sensor), two axes (2-D sensor), or more axes. Both thebutton-type and touchpad-type sensors can also optionally be configuredto provide additional information such as some indication of the force,duration, or amount of capacitive coupling associated with the input.Examples of 1-D and 2-D touchpad-type sensors based on capacitivesensing technologies are described in United States PublishedApplication 2004/0252109 A1 to Trent et al. and U.S. Pat. No. 5,880,411,which issued to Gillespie et al. on Mar. 9, 1999. Such sensors can bereadily found, for example, in input devices of electronic systemsincluding handheld and notebook-type computers.

A user generally operates capacitive input devices by placing or movingone or more fingers, styli, and/or other objects near a sensing regionof the sensor(s) located on or in the input device. This creates acapacitive effect upon a carrier signal applied to the sensing regionthat can be detected and correlated to positional information (such asthe position(s) or proximity or motion or presences or similarinformation) of the stimulus/stimuli with respect to the sensing region.This positional information can in turn be used to select, move, scroll,or manipulate any combination of text, graphics, cursors, highlighters,and/or other indicators on a display screen. This positional informationcan also be used to enable the user to interact with an interface, suchas to control volume, to adjust brightness, or to achieve any otherpurpose.

Although capacitance sensors have been widely adopted, sensor designerscontinue to look for ways to improve the sensors' functionality andeffectiveness. In particular, it is continually desired to simplify thedesign and implementation of such sensors. Moreover, a need continuallyarises for a highly versatile yet low cost and easy to implement sensordesign. In particular, a need exists for a sensor design scheme that isflexible enough to be easily implemented across a wide variety ofapplications yet powerful enough to provide accurate capacitancesensing, while at the same time remaining cost effective.

Accordingly, it is desirable to provide systems and methods for quickly,effectively and efficiently detecting a measurable capacitance.Moreover, it is desirable to create a scheme that can be implementedusing readily available components, such as standard ICs,microcontrollers, and discrete components. Other desirable features andcharacteristics will become apparent from the subsequent detaileddescription and the appended claims, taken in conjunction with theaccompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

Methods, systems and devices are described for detecting a measurablecapacitance using charge transfer techniques that are implementable onmany standard microcontrollers without requiring external, active analogcomponents. According to various embodiments, a charge transfer processis performed two or more times. The charge transfer process comprisesapplying a pre-determined voltage to the measurable capacitance, andthen allowing the measurable capacitance to share charge with a filtercapacitance through a passive impedance that remains coupled to both themeasurable capacitance and to the filter capacitance throughout theperiods of applying of the pre-determined voltage and of allowing of themeasurable capacitance to share charge. The value of the measurablecapacitance can then be determined as a function of a representation ofa charge on the filter capacitance and the number of times that thecharge transfer process was performed. The number of times that thecharge transfer process is executed can be pre-established or be basedon the representation of the charge reaching some threshold. Therepresentation of the charge on the filter capacitance can be obtainedby a measuring step that produced a single-bit or multi-bit measurement.These steps can be repeated, and the results of the measuring step canbe stored and/or filtered as appropriate.

Using the techniques described herein, a capacitance detection schememay be conveniently implemented using readily available components, andcan be particularly useful in sensing the position of a finger, stylusor other object with respect to a capacitive sensor implementing button,slider, cursor control, or user interface navigation function(s), or anyother functions.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present invention will hereinafter be describedin conjunction with the following drawing figures, wherein like numeralsdenote like elements, and:

FIGS. 1A-D are block diagrams of exemplary implementations ofcapacitance sensors;

FIG. 2A-B are timing diagrams showing exemplary techniques for operatinga capacitance sensor such as that shown in FIG. 1B;

FIG. 3A-B are timing diagrams showing an alternate technique foroperating a capacitance sensor such as that shown in FIG. 1B;

FIGS. 4A-C are block diagrams of alternate embodiments of capacitancesensors;

FIG. 5 is a timing diagram showing an exemplary technique for operatinga capacitance sensor such as the sensor shown in FIG. 4A;

FIG. 6 is a block diagram showing an alternate embodiment of amulti-channel capacitance sensor incorporating a guard electrode;

FIG. 7 is a block diagram showing another alternate embodiment of amulti-channel capacitance sensor;

FIG. 8 is a flowchart of an exemplary technique for detectingcapacitance using switched charge transfer techniques;

FIG. 9 is a schematic diagram of a proximity sensor device using acapacitance sensor coupled with an electronic system.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

According to various exemplary embodiments, a capacitance detectionand/or measurement circuit can be readily formulated using a passiveelectrical network and one or more switches. In a typicalimplementation, a charge transfer process is executed for two or moreiterations in which a predetermined voltage is applied to the measurablecapacitance using one or more of the switches, and in which themeasurable capacitance is allowed to share charge with a filtercapacitance in the passive network. (The filter capacitance can also bereferred to as an “integrating capacitance” or an “integrating filter.”)With such a charge transfer process, a plurality of applications of thepre-determined voltage and the associated sharings of charge influencethe voltage on the filter capacitance. The charge transfer process thuscan be considered to roughly “integrate” charge onto the filtercapacitance over multiple executions such that the “output” voltage ofthe filter capacitance is filtered. After two or more iterations of thecharge transfer process (although some embodiments may use only oneiteration), the representation of charge on the filter capacitance isread to determine the measurable capacitance. The representation of thecharge on the filter capacitance can be a voltage on the filtercapacitance, such as a voltage on a node of the circuit that indicatesthe voltage across the filter capacitance. The voltage on the filtercapacitance can also be the voltage across the filter capacitanceitself. The measuring of the representation of the charge on the filtercapacitance can entail comparison with one or more thresholds togenerate a single or multi-bit reading. The measuring can entail use ofmulti-bit analog-to-digital circuitry to generate a multi-bit measure ofthe representation of the charge.

Using these techniques, capacitive position sensors capable of detectingthe presence or proximity of a finger, stylus or other object can bereadily formulated. Additionally, various embodiments described hereincan be readily implemented using only conventional switching mechanisms(e.g. those available through the I/Os of a control device) and passivecomponents (e.g. one or more capacitors, resistors, inductors, and/orthe like), without the need for additional active electronics that wouldadd cost and complexity. As a result, the various schemes describedherein may be conveniently yet reliably implemented in a variety ofenvironments using readily-available and reasonably-priced components,as described more fully below.

The measurable capacitance is the effective capacitance of any signalsource, electrode, or other electrical node having a capacitancedetectable by a capacitive sensing system. For capacitive proximitysensors and other input devices accepting input from one or morefingers, styli, and/or other stimuli, measurable capacitance oftenrepresents the total effective capacitance from a sensing node to thelocal ground of the system (“absolute capacitance”). The total effectivecapacitance for input devices can be quite complex, involvingcapacitances, resistances, and inductances in series and in parallel asdetermined by the sensor design and the operating environment. In othercases, measurable capacitance may represent the total effectivecapacitance from a driving node to a sensing node (“transcapacitance”).This total effective capacitance can also be quite complex. However, inmany cases the input can be modeled simply as a small variablecapacitance in parallel with a fixed background capacitance.

In input devices using capacitance sensors, the measurable capacitanceis often the variable capacitance exhibited by a sensing electrode ofthe capacitance sensor. The capacitance sensor may include multiplesensing electrodes, and each sensing electrode may be associated with ameasurable capacitance. With an exemplary “absolute capacitance” sensingscheme, the measurable capacitance would include the capacitive couplingof the sensing electrode(s) to one or more input objects, such as anycombination of finger(s), styli, or other object(s), that are closeenough to the sensing electrode(s) to have detectable capacitivecoupling with the sensing electrode(s). With an exemplary“transcapacitance” sensing scheme, the measurable capacitance wouldinclude the capacitive coupling of the sensing electrode(s) to one ormore driving electrodes. This coupling to the input object(s) (for the“absolute capacitance” scheme) or between electrodes (for the“transcapacitance” scheme) changes as the electric field is affected bythe input object(s). Thus, the value of the measurable capacitance canbe used to ascertain information about the proximity, position, motion,or other positional information of the input object(s) for use by thecapacitive input device or by any electronic system in communicationwith the capacitive input device.

The value of the pre-determined voltage applied to the measurablecapacitance is often known, and often remains constant. For example, thepre-determined voltage can be a single convenient voltage, such as apower supply voltage, a battery voltage, a digital logic level, aresistance driven by a current source, a divided or amplified version ofany of these voltages, and the like. However, the pre-determined voltagecan also be unknown or variable, so long as the pre-determined voltageremains ratiometric with the measurement of the charge on the filtercapacitance. For example, a capacitance sensing scheme can involveresetting the filter capacitance to a reset voltage, and also involvemeasuring a voltage across the filter capacitance by comparing thevoltage (as relative to the reset voltage) on one side of the filtercapacitance with a threshold voltage (also as relative to the resetvoltage); with such a sensing scheme, the difference between thepre-determined voltage and the reset voltage, and the difference betweenthe threshold voltage and the reset voltage, should remain roughlyproportional to each other, on average over the execution(s) of thecharge transfer process leading to the determination of the measurablecapacitance. Thus, the threshold used to measure the change in voltageon the filter capacitance will be proportional to the change in voltageon the filter capacitance due to the charge shared from the measurablecapacitance to the filter capacitance during the execution(s) of thecharge transfer process for a determination of the measurablecapacitance. In particular, where the pre-determined voltage is V_(cc)and the reset voltage is GND, the threshold voltage can be ratiometricfor a CMOS input threshold, for example (½)*(V_(cc)−GND).

Turning now to the figures and with initial reference to FIG. 1A, anexemplary capacitance sensor 100 for determining a measurablecapacitance 112 suitably includes a passive impedance 105 coupled with afilter capacitance 110. Although sensor 100 is driven using switches101, 103, measurable capacitance 112, filter capacitance 110, andpassive impedance 105 still form a passive electrical network thatincludes no active elements. Passive impedance 105 is provided by one ormore non-active electronic components, such as any combination ofcapacitance(s), inductance(s), resistance(s), and the like.Capacitances, resistances, and inductances can be provided by anycombination of capacitive, resistive, and inductive elements,respectively. Some elements exhibit more than one impedance property,such as having both resistive and inductive properties; these elementswould thus provide both a resistance and an inductance to the network inwhich it is used. In various embodiments, passive impedance 105 is aresistance provided by a network or one or more resistors. Additionally,passive impedance 105 can include non-linear components such as diodes.Impedance 105 is generally designed to have an impedance that is largeenough to prevent significant charge leaking into filter capacitance 110during the applying of the pre-determined voltage to the measurablecapacitance 112, as described more fully below. In various embodiments,impedance 105 may be a resistance on the order of a hundred kilo-ohms ormore, although other embodiments may exhibit widely different impedancevalues.

Filter capacitance 110 is coupled to node 107 and to passive impedance105 at node 115. Node 107 can be coupled to a suitable voltage (groundis shown in FIG. 1A although another reference voltage can be used).Filter capacitance 110 can be provided by one or more capacitors (suchas a collection of any number of discrete capacitors) configured toaccept charge transferred from measurable capacitance 112. Although theparticular filter capacitance value selected will vary from embodimentto embodiment, the capacitance value of filter capacitance 110 willtypically be an order of magnitude, and often several orders ofmagnitude, greater than the capacitance value of the measurablecapacitance 112. For example, filter capacitance 110 may be designed onthe order of several nanofarads or so, whereas measurable capacitance112 could be on the order of picofarads. As one example that will bedescribed in greater detail below, the filter capacitance 110 isselected such that the time constant of the RC circuit created by filtercapacitance 110 and passive impedance 105 is greater than the durationof the pulses used to apply the pre-determined voltage to measurablecapacitance 112.

The time constant of measurable capacitance 112 with passive impedance105 is also preferably greater than the duration of the pre-determinedvoltage applied by the pulses to measurable capacitance 112. This is sothat the charge added to filter capacitance 110 during the chargetransfer process comes mostly from the charge stored on the measurablecapacitance 112 and shared with filter capacitance 110, and less fromany flow of current through passive impedance 105 during the applying ofthe pre-determined voltage. Since filter capacitance 110 is often ordersof magnitude greater than measurable capacitance 112 in order to provideadequate capacitance-sensing resolution, it follows that its timeconstant with passive impedance 105 is also orders of magnitude greaterthan the duration of pre-determined voltage applied by the pulses. Thus,a relatively large time constant of the RC circuit allows the chargeleakage to the filter capacitance 110 during the applying of thepre-determined voltage to be relatively small.

Sensor 100 also includes a switch 103 in parallel with filtercapacitance 110 and coupled to nodes 115 and 107. Switch 103 can beclosed to reset the charge on filter capacitance 110 before performingthe charge transfer processes for a determination of a value of themeasurable capacitance 112. In this case, closing switch 103 clears thecharge on filter capacitance 110.

Other options for resetting filter capacitance 110 are readilyavailable. For example, switch 103 can couple node 113 to a voltage suchas ground (instead of nodes 107 and 115) such that closing switch 103would reset the charge on filter capacitance 110 through passiveimpedance 105. Such a configuration may be implemented using a singledigital I/O of a controller (such as shown in FIG. 4A). However, thisconfiguration would reset with the time constant associated with passiveimpedance 105 and filter capacitance 110, and thus require a reset timegreater than placing switch 103 in parallel with filter capacitance 110.

Operation of capacitance sensor 100 suitably involves a charge transferprocess and a measurement process facilitated by the use of one or moreswitches 101, 103. Switches 101, 103 may be implemented with any type ofdiscrete switches, buffered integrated circuits, field effecttransistors and/or other switching constructs, to name just a fewexamples. Alternatively, switches 101, 103 can be implemented withinternal logic/circuitry of a controller coupled to an output pin of thecontroller, as will be discussed in greater detail below. The output pinof the controller may also be coupled to internal logic/circuitrycapable of providing input functionality, such that switches 101, 103can be implemented using one or more I/Os of a controller.

The charge transfer process, which is typically repeated two or moretimes, suitably applies a pre-determined voltage (convenient voltagesfor the pre-determined voltage include a power supply voltage, a batteryvoltage, and a logic signal) to the measurable capacitance 112, and thenallows measurable capacitance 112 to share charge with filtercapacitance 110 as appropriate. In the example shown in FIG. 1A, closingswitch 101 applies the pre-determined voltage to measurable capacitance112 and opening switch 101 ceases the application of the pre-determinedvoltage to measurable capacitance 112. Circuit 100 illustrates aconfiguration where switch 101 is used to apply a voltage when it isclosed and the application of the voltage ceases when switch 101 it isopen. However, switches can be used to apply voltages when opened orclosed, or used to apply a first voltage when open and a differentvoltage when closed. Thus, switch 101 is used to apply thepre-determined voltage in pulses or other waveforms that have arelatively short period in comparison to the RC time constant associatedwith impedance 105 and measurable capacitance 112 or filter capacitance110 to help prevent excessive current leakage through impedance 105during the applying of the pre-determined voltage. Leakage of chargethrough impedance 105 can be detrimental to sensor accuracy and/orresolution, since it is often difficult, if not practically impossible,to control or account for the charge leakage in measuring therepresentation of the charge on the filter capacitance 110 ordetermining a value of the measurable capacitance 112. Charge leakagethrough parasitic or stray impedances in addition to impedance 105 canalso be detrimental to sensor performance, and this effect can bereduced by having a shorter application of the pre-determined voltage.

After applying the pre-determined voltage to measurable capacitance 112,the applying of the pre-determined voltage for that performance of thecharge transfer process is ended by opening switch 101 and themeasurable capacitance 112 is allowed to share charge with filtercapacitance 110. During charge sharing, charge travels betweenmeasurable capacitance 112 and filter capacitance 110 through passiveimpedance 105. Passive impedance 105 remains coupled to the measurablecapacitance 112 and the filter capacitance 110 during both the chargingperiod (when the pre-determined voltage is applied to charge ordischarge the measurable capacitance) and the allowing to share period(when the application of the pre-determined voltage is stopped).Although FIG. 1A shows a particular configuration for accomplishing thischarge sharing, the sharing of charge through the passive impedance canbe done a multitude of ways, and other circuits can be used withoutdeparting from the principles disclosed herein. For example, theelectrical path from measurable capacitance 112 to ground in FIG. 1A(when the predetermined voltage is not applied and charge sharing isallowed) includes both impedance 105 and filter capacitance 110 inseries. Since the impedance 105 and filter capacitance 110 are in seriesand the principle of the additivity of impedances applies, the relativepositions of filter capacitance 110 and impedance 105 could be exchangedfor this sensor 100 embodiment without altering the charge transferprocess and the operation of the circuit.

To allow measurable capacitance 112 to share charge with the passivenetwork, no action may be required other than to stop applying thepre-determined voltage and to pause (also “delay”) for a time sufficientto allow charge to transfer between the measurable capacitance and thefilter capacitance. This is true, for example, for the embodiment shownin FIG. 1A, where the pause time required for the charge to sharebetween measurable capacitance 112 and filter capacitance 110 isdetermined by the time constant of the circuit including the measurablecapacitance 112, the passive impedance 105, and the filter capacitance110. In various embodiments, the pause time required may be relativelyshort (e.g. if the filter capacitance 110 is connected to the measurablecapacitance 112 with a small resistance in series). In otherembodiments, the delay required may be longer such that a lengthierpause time may be needed (e.g. if the filter capacitance 110 isconnected to the measurable capacitance 112 with more significantpassive impedance 105 in series). In other embodiments, allowing chargeto transfer may involve actively actuating one or more switchesassociated with a controller to couple components external to thepassive network, and/or taking other actions as appropriate. In suchembodiments, the passive impedance 105 may be made smaller.

After performing one or more executions of the charge transfer processthat includes the applying of the pre-determined voltage and theallowing of the measurable capacitance 112 to share charge with thefilter capacitance 110, a representation of the charge on the filtercapacitance 110 can be measured. The representation of the charge onfilter capacitance 110 can be conveniently taken as the voltage at node115 for the embodiment shown in FIG. 1A. The representation of thecharge on filter capacitance 110 can also be taken as the voltage atnode 113, but the voltage across passive impedance 105 must be accountedfor in that case. However, the voltage across passive impedance 105 maybe negligible for some embodiments and at certain times; if measurementsare taken at these certain times for these embodiments, then the voltageat node 113 would then effectively be equivalent to the voltage at node115. For example, if passive impedance 105 is a resistance, thennegligible charge is stored on passive impedance 105; in addition, ifmeasurable capacitance 112 and filter capacitance 110 have completedsharing, then insignificant current flows through passive impedance 105.In such a case, the voltage across passive impedance 105 is practicallyzero.

The measurement of the representation of the charge on filtercapacitance 110 can be achieved by using a simple comparison with athreshold (such as by using a comparator to produce a single-bitmeasurement) or by more complex circuitry (such as by using a multi-bitADC to produce a multi-bit measurement). When a threshold is used toproduce a single-bit measurement, typically multiple measurements aretaken to ascertain the number of executions of the charge transferprocess needed for the representation of the charge to cross thisthreshold. This number of executions necessary can be used along withknown values (e.g. the threshold, the value of filter capacitance 110,etc.) to determine a value of the measurable capacitance 112. When amulti-bit ADC is used to produce a higher resolution measurement, fewermeasurements can be taken (one single measurement may be sufficient ifthe ADC provides sufficient resolution) and the number of executionsbefore taking the measurement(s) can be pre-established. The multi-bitADC measurement can be used along with known values and thepre-established number to determine the value of the measurablecapacitance 112. Additional charge transfer processes can be performedafter obtaining the measurement(s) for determining the value of themeasurable capacitance 112 to bring the filter capacitance 110 to areset state, or for convenience of design if the filter capacitance 110is reset using a different method. These additional charge transferprocesses may be especially useful for sensing multiple measurablecapacitances.

As stated above, switches 101, 103 can be implemented with separate,discrete switches, or with the internal logic/circuitry coupled to anoutput or an input/output (I/O) of a controller. Turning now to FIG. 1B,a second exemplary capacitance sensor 150 is illustrated. Thecapacitance sensor 150 uses a controller 102 with I/Os 104 and 106 toprovide switching functionality. I/O 104 can provide the switchingassociated with switch 101 of FIG. 1A. I/O 106 can provide switchingfunctionality to reset the filter capacitance, but the effect differsfrom that associated with switch 103. In the case of the embodiment ofFIG. 1B, I/O 106 is used to provide a reference voltage as does switch103, but the filter capacitance 110 is reset to having a set amount ofnonzero charge on filter capacitance 110 since node 107 is coupled to avoltage that typically can not be supplied by I/O 106. Digital I/Os ofcontrollers are typically capable of switchably applying one or morelogic values and/or a “high impedance” or “open circuit” value. Thelogic values may be any appropriate voltage or other signal. Forexample, a logic “high” or “1” value could correspond to a “high”voltage (e.g. +V_(cc), which can be +5 volts for some controllers, orthe like), and a logic “low” or “0” value could correspond to acomparatively “low” voltage (e.g. ground or 0V). Thus, the particularsignals selected and applied using I/Os 104, 106 can vary significantlyfrom implementation to implementation depending on the particularcontroller 102 selected. Thus, one advantage of these embodiments usingcontroller I/Os is that a very flexible capacitance sensor 150 can bereadily implemented using only passive components (e.g., passiveresistance 105, filter capacitance 110) in conjunction with aconventional controller 102 such as a microcontroller, digital signalprocessor, microprocessor, programmable logic array, applicationspecific integrated circuit and/or the like. A number of thesecontroller products are readily available from various commercialsources including Microchip Technologies of Chandler, Ariz.; FreescaleSemiconductor of Austin, Tex.; and Texas Instruments of Dallas, Tex.,among others

Further, in some embodiments, the controller 102 includes digital memory(e.g. static, dynamic or flash random access memory) that can be used tostore data and instructions used to execute the various charge transferprocessing routines for the various capacitance sensors containedherein. Because impedance 105 and filter capacitance 110 are staticallyconnected, the only physical action that needs take place during sensoroperation involves manipulation of signal levels at I/Os 104 and 106.Such manipulation may take place in response to software, firmware,configuration, or other instructions contained in controller 102.

In some embodiments, the filter capacitance 110 is coupled to aneffectively constant voltage such as ground at node 107 such as shown inFIG. 1A. In other embodiments, the filter capacitance 110 is coupled toa varying voltage that improves the performance of the capacitancedetection, such as shown in FIG. 1B. The exemplary embodiment shown inFIG. 1B includes such an optional compensation circuit 125 thatcompensates capacitance sensor 150 for fluctuations in power supplyvoltages, thereby providing improved resistance to power supply voltagenoise effects. Compensation circuit 125 typically couples the side offilter capacitance 110 opposite the measurable capacitance 112 to eitheror both power supply rails (coupling to +V_(cc) and ground is shown inFIG. 1B) associated with the implementation of capacitance sensor 150.Although FIG. 1B shows compensation circuit 125 compensating only onefilter capacitance 110, the same compensation circuit can be coupled tomultiple filter capacitance 110 at node 107. Thus, compensation circuit125 can easily be used to compensate sensors with multiple sensingchannels and multiple filter capacitances. With the configuration shownin FIG. 1B, fluctuations in the supply rails (also “supply voltageripple”) induce similar fluctuations in the voltage at node 107, andtherefore can be used to compensate for fluctuations in thresholdsassociated with controller 102 induced by the same supply voltageripple.

The exemplary compensation circuit 125 shown in FIG. 1B, includes twoimpedances 127 and 129 configured in an impedance divider arrangementbetween the two supply voltages of +V_(cc) and ground. An impedancedivider is composed of two passive impedances in series, where eachpassive impedance is coupled to at least two nodes. One of these nodesis common to both impedances (“a common node” to which both impedancesconnect.) The common node serves as the output of the impedance divider.The output of the impedance divider is a function of the voltages and/orcurrents applied at the “unshared” nodes (the nodes of the twoimpedances that are not the common node) over time. A simple example ofan impedance divider is a voltage divider composed of two capacitancesor two resistances. More complex impedance dividers may have unmatchedcapacitances, resistances, or inductances in series or in parallel. Animpedance may also have any combination of capacitive, resistive, andinductive characteristics.

For compensation circuit 125 shown in FIG. 1B, the impedance divider canbe a voltage divider formed from two resistances or two capacitancescoupled to +V_(cc) and ground. The impedance divider of circuit 125 hasa “common node” coupled to the filter capacitance 110 at node 107.Resistive versions of impedances 127 and 129 can comprise resistors toform a resistive divider network, and capacitive versions of impedances127 and 129 can comprise capacitors to form a capacitive dividernetwork. By selecting appropriate values for impedances 127 and 129,filter capacitance 110 can be biased toward any voltage that liesbetween the two supply voltages. Moreover, variations in supply voltagewill be automatically compensated by the compensation circuit 125. Thisis because such a voltage divider provides a voltage that reflects thefluctuations in power supply voltage without significant lag. Althoughadvantageous for some embodiments, the use of a compensation circuit 125may not be desirable in all embodiments.

FIG. 1D shows another sensor circuit 195 that demonstrates anothermethod of coupling the filter capacitance to a varying voltage thatimproves the performance of the capacitance detection. In circuit 195,voltage compensation is achieved by using a combination filtercapacitance 110 formed from a capacitive impedance divider staticallycoupled to both power supply rails. This combination filter capacitance110 includes a first filter capacitance 1102 statically coupled to afirst power supply rail (+V_(cc) is shown in FIG. 1D) and a secondfilter capacitance 1104 statically coupled to the other power supplyrail (GND is shown in FIG. 1D). The first and second filter capacitances1102 and 1104 are coupled to each other at their common node, which isalso node 115. Node 115 is further coupled to passive impedance 105 andI/O 106. The passive impedance 105 is also coupled to I/O 104 andmeasurable capacitance 112 at node 113. Overall, the configuration ofcircuit 195 is very similar to the configuration of circuit 150 (FIG.1B) without compensation circuit 125; however, circuit 195 has a splitfilter capacitance that couples to both power supply rails.

The operation of circuit 195 can be very similar to the operation ofcircuit 150. I/O 104 can apply the predetermined voltage to measurablecapacitance 112 by providing a logic value (e.g. a logic “high”). I/O104 can then be held at high impedance to allow charge sharing betweenmeasurable capacitance 112 and both capacitances 1102-1104 ofcombination filter capacitance 110. I/O 106 (or some other circuitry)can be used to measure the voltage at node 115, which is stillrepresentative of the charge on combination filter capacitance 110 bybeing representative of the total charge on capacitances 1102-1104. I/O106 can provide a reset signal (e.g. a logic “low”) to reset the chargeon both capacitances 1102-1104 after the appropriate number of chargetransfer processes has been performed. With the configuration shown inFIG. 1D, fluctuations in the supply rails induce similar fluctuations inthe voltages to which combination filter capacitance 110 is referenced(+V_(cc) for capacitance 1102 and GND for capacitance 1104). Therefore,the embodiment of circuit 195 can compensate for fluctuations inthresholds associated with controller 102 that are induced by the samesupply voltage ripple.

Since the circuit 195 achieves compensation by using the filtercapacitance, it has the advantage over compensation circuit 125 of fewercomponents when only one or two filter capacitances are needed by thesensor. In many cases, the compensation illustrated by circuit 195 canbe shared by multiple sensing channels that share the same filtercapacitance (e.g. circuit 440 of FIG. 4C, discussed below). However, thesplit-filter-capacitance method illustrated by FIG. 1D is more difficultto share across multiple sensing channels than compensation circuit 125of FIG. 1B, since sharing the compensation would share the filtercapacitance.

With reference to FIG. 2A, an exemplary timing scheme 200 is shown thatwould be suitable for operating capacitance sensor 150 of FIG. 1B.Specifically, FIG. 2A illustrates the voltages associated with I/Os 104and 106, and at nodes 113 and 115. Trace 230 shows an exemplary set ofcharging voltage pulses 210 that can be provided to measurablecapacitance 112 using I/O 104. The charging voltage pulses 210 includeboth logic low (0) output portions 209 and logic high (1) outputportions 201. In the embodiment shown in FIGS. 1B, 2A, the logic highoutput portions apply the pre-determined voltage to the measurablecapacitance 112 via node 113 such that the pre-determined voltage is thevoltage associated with a logic high output (which is +V_(cc) for atypical controller 102). Thus, the pre-determined voltage is appliedusing the appropriate switch (e.g. via circuitry internal to controller102 that produces the appropriate signal output on I/O 104 in FIG. 1B).The logic high portions 201 of charging voltage pulses 210 are generallyselected to have a period shorter than the response time of the RCcircuit that includes impedance 105 and filter capacitance 110 such thatany charge leakage to the filter capacitance 110 during the applying ofthe pre-determined voltage will be negligible.

In the embodiment shown in FIG. 2A, the charging voltage pulses 210 alsoinclude relatively brief logic low output portions 209 (which is GND fora typical controller 102, although other controllers may output otherlogic values). Logic low output portions 209 provide an “opposing”voltage that precedes the logic high output portions 201 that apply thepre-determined voltage. The “opposing” voltage has a magnitude oppositethat of the pre-determined voltage and helps compensate for currentleaking through impedance 105 during the charge transfer process by“current cancelling.” The durations of the logic output portions 209 arechosen so the amount of parasitic charge removed by driving the opposingvoltage is mostly equal to the amount of parasitic charge added byapplying the pre-determined voltage. In the charging pulses 210 of FIG.2A, “opposing” voltage is applied for about the same duration as theapplication of the pre-determined voltage. Such “current cancelling” isan optional feature that is not required in all embodiments.

Trace 232 of FIG. 2A illustrates an exemplary set of reset signals 220that can be provided by I/O 106. These reset signals 220 can provide areset voltage to the filter capacitance 110 at node 115 to reset thecharge on the filter capacitance 110. The reset signals 220 are providedto node 115 through a switch (e.g. via circuitry internal to controller102 that produces the appropriate signal output on I/O 106 in FIG. 1B).In the illustrated example of FIG. 2A, the reset signals 220 compriselow voltage outputs that reset the charge contained on filtercapacitance 110. This reset signal may be provided periodically,aperiodically, or otherwise, and/or may not be provided at all in someembodiments, as described more fully below. For example, a particularreset signal 220 can be applied to node 115 after the voltage at node115 (which is a representation of the charge on the filter capacitance110) passes a threshold; this is shown in timing chart 200 with thereset signals 220A-C being provided after the voltage at node 115 (shownby trace 236) passes a threshold voltage V_(TH).

Between the applications of the charging voltage pulses 210, themeasurable capacitance 112 is allowed to share charge with filtercapacitance 110 by holding I/O 104 to a high impedance state (Z). Theembodiment of timing chart 200 shows the pre-determined voltage beingapplied by logic highs, such that the measurable capacitance 112 chargesduring the applying of the pre-determined voltage and discharges throughimpedance 105 to filter capacitance 110 during charge sharing. FIG. 2Aillustrates an exemplary voltage trace 234 of the voltage at node 113that shows the result of each charging voltage pulse 210 on the voltageat node 113. Specifically, voltage trace 234 illustrates how eachopposing voltage portion 209 of a charging pulse 210 drives the voltageon measurable capacitance 112 to the opposing voltage (which is shown asground in FIG. 2A), how each pre-determined voltage portion 201 of acharging pulse drives the voltage on measurable capacitance 112 to thepre-determined voltage (which is shown as +V_(cc) in FIG. 2A), and howcharge sharing between the measurable capacitance 112 and filtercapacitance 110 brings the voltages on the measurable capacitance 112(at node 113) and filter capacitance 110 (at node 115) close to eachother. Thus, voltage trace 234 shows the results as each chargingvoltage pulse 210 removes charge by applying ground to node 113 andcharges the measurable capacitance 112 by applying +V_(cc) and theresults as the measurable capacitance 112 shares charge with the filtercapacitance 110.

The resulting voltage on filter capacitance 110 (i.e. the voltage atnode 115 for the embodiment shown in FIGS. 1B, 2A), is illustrated inFIG. 2A as voltage trace 236. Voltage trace 236 exhibits slight dropsand rises in response to the driving of the opposing voltage and thepre-determined voltage in “current cancelling.” Voltage trace 236 alsoshows the gradual increase in voltage at node 115 as the filtercapacitance 110 charge shares with measurable capacitance 112 overperformances of the charge transfer process.

Voltage traces 234 and 236 both show how the measurable capacitance 112is charged relatively quickly with each application of thepre-determined voltage, while the relatively large time constant of themeasurable capacitance 112 or filter capacitance 110 and passiveimpedance 105 that forms the passive network causes a relatively slowerdischarging of the filter capacitance 110. Typically, the sharing periodis sized to be long enough for the voltages at node 113 and 115 toroughly equalize, such that any current still flowing between measurablecapacitance 112 and filter capacitance 110 is negligible.

Several different methods can be used to determine the measurablecapacitance 112 from the voltage at the filter capacitance 110. In onemethod, the voltage at node 115 is compared to an appropriate thresholdvoltage (V_(TH)) to provide a single bit analog-to-digital (A/D)conversion of the voltage on the filter capacitance 110. (As discussedearlier, the voltage at node 113 may also be used as a representation ofthe charge on filter capacitance 110, and can be measured using I/O 104.However, for this example, the voltage at node 115 is used.) Thisvoltage comparison can be performed by a comparator coupled to node 115.For example, the controller 102 of FIG. 1B can include an analog voltagecomparator function coupled to I/O 106. Alternatively, the comparatorfunction can be performed by the input circuitry of an ordinary digitalI/O pin 106, in which case the threshold V_(TH) is typically thepredetermined threshold of the digital input buffer (such as a CMOSthreshold). Thus I/O 106 can be used to compare the voltage at node 115to the threshold voltage and thus obtain a representation of the chargeon filter capacitance 110. In one embodiment, this threshold voltage isroughly equivalent to the midpoint between the high and low logicvalues.

The embodiment shown in FIGS. 1B, 2A can use this comparator-typemethod, and the timing chart 200 illustrates this method with the dashedV_(TH) lines indicating a threshold voltage level V_(TH) and the points222 indicating I/O 106 reading times. As shown in trace 236, the chargetransfer process of applying charging pulses 210 to the measurablecapacitance 112 and allowing charge sharing by the measurablecapacitance 112 with filter capacitance 110 repeats until the voltage atnode 115 is detected to exceed the threshold voltage V_(TH). I/O 106 ofcontroller 102 has input functionality, and can be read to measure thevoltage at node 115 by comparing it with an input threshold of I/O 106.This measuring and ascertaining of the voltage on filter capacitance 110can occur for some or all performances of the charge transfer process.Timing chart 200 of FIG. 2A shows the reading of I/O 106 at points222A-D, which include only some of the performances of the chargetransfer process.

After the voltage at node 115 exceeds the threshold (indicated by points203A,B on trace 236, and after this crossing of the threshold isdetected by the system through reading of I/O 106 at points 222B,D),reset signals 220B,C are applied to node 115 to reset the charge onfilter capacitance 110. Although sometimes only one performance of thecharge transfer process is needed to cross the threshold, typically tensor hundreds or more performances of the charge transfer process areinvolved. Timing chart 200 shows the threshold being passed after onlyfour performances of the charge transfer process for convenience ofexplanation. By ascertaining the number of charge transfer processesperformed until the voltage on filter capacitance 110 (i.e. the voltageat node 115 for the embodiment shown in FIGS. 1B, 2A) exceeds thethreshold voltage V_(TH), a value of the measurable capacitance 112 canbe effectively determined. That is, the number of charge/dischargecycles of measurable capacitance 112 performed to produce a known amountof charge on capacitance 110 (as indicated by the voltage at node 115passing a known voltage threshold V_(TH)) can be effectively correlatedto the actual capacitance of measurable capacitance 112.

This comparator-type method can also be implemented with any combinationof circuitry internal and/or external to controller 102 as appropriate.Many variations of this comparator-type method also exist and arecontemplated. For example, multiple thresholds can be provided using amultitude of reference voltages for one or more comparators, using aspecialized input of the controller 102, or using an input of thecontroller having hysteresis (e.g. a Schmitt trigger type input). Ifmultiple thresholds are used, the charge transfer process can alsochange as different thresholds are reached. For example, the chargetransfer process can be configured to transfer relatively larger amountsof charge to reach coarser thresholds if the thresholds are not evenlyspaced, or first thresholds if there is a multitude of thresholds thatall must be crossed. The number of performances of each type of chargetransfer process needed to cross the last threshold crossed can be usedto determine the value of the measurable capacitance 112; additionalinformation concerning the crossing of other thresholds can also be usedto refine the determination.

In other embodiments, alternative methods of determining the value ofthe measurable capacitance are used. In another method, a directmulti-bit measurement of the voltage at node 115 is taken and used todetermine the capacitance of the measurable capacitance 112. Forexample, the controller 102 can include a high resolutionanalog-to-digital function that allows more accurate measurement ofvoltage 115. In such embodiments, the charge transfer process canexecute for a pre-set number of times, after which a multi-bit value ofthe voltage at node 115 is measured. After the measurement and thepre-set number of times, the charge on filter capacitance 110 can bereset for the next cycle of executions of the charge transfer processes.

The embodiment shown by FIGS. 1B, 2A can use this multi-bit method, andthe timing chart 200 of FIG. 2A illustrates this. Ignoring the voltagethreshold V_(TH) lines and the reading points 222A,C, voltage trace 236shows four as the pre-established number of performances of the chargetransfer process and illustrates the measuring of the voltage at node115 after four executions of the charge transfer process at points205A-B. As in the comparator-type method, the multi-bit measurement canbe taken using any combination of circuitry internal and/or external tocontroller 102. For example, I/O 106 can be used directly if it hasmulti-bit input functionality. Alternatively, another input of 102,another controller with ADC capability, or an external ADC or some othercircuitry can be coupled to controller 102 and used to measure. Thevalue of measurable capacitance 112 is then determined from the measuredvoltage on the filter capacitance 110 (the voltage at node 115 for theembodiment shown in FIG. 1B).

Many variations of this ADC-type method exist and are contemplated. Forexample, multi-bit measurements can be taken at multiple times in a setof charge transfer processes performed between resets of the filtercapacitance 110. As another example, the number of executions of thecharge transfer process does not have to be pre-set, such that both thenumber of charge transfer processes performed and the voltage at node115 are tracked to produce a value of the measurable capacitance 112.

Other methods can be used to determine the measurable capacitance 112from the voltage at the filter capacitance 110, and FIG. 2B illustratessome example voltage traces on filter capacitance 110 (e.g. voltage atnode 115) that may result. For example, one method entails performingthe charge transfer process for a pre-set number of times, and thendrawing charge from filter capacitance 110 using a current source (trace252) or a discharge circuit with a known time response, such as a firstorder response (trace 254). The time needed for the charge on filtercapacitance 110 to fall to a known value such as zero can be monitoredand quantized to produce a single or a multi-bit measurement of therepresentation of the charge on the filter capacitance 110. Thismeasurement can be used along with the pre-set number in determining thevalue of measurable capacitance 112. With such methods, the filtercapacitance 110 can be left at the known value after measurement, suchthat no separate reset signal needs to be applied.

Many changes could be made to the basic structures and operations shownin FIGS. 1A-B and 2. The timing scheme 200 shown in FIG. 2A assumes a“positive” transfer of charge from measurable capacitance 112 to filtercapacitance 110, for example, whereas equivalent embodiments could bebased upon sharing of charge in the opposite direction. (That is, chargecould be placed on filter capacitance 110 during reset, and this chargeis removed and drawn through impedance 105 to measurable capacitance 112during sharing. This can be done by applying a high reset voltage tonode 115, and then using charging pulses 210 that apply a lowpre-determined voltage to measurable capacitance 112 such that sharingbetween the measurable capacitance 112 and filter capacitance 110discharges filter capacitance 110 and lowers the voltage at node 115.)Traces 256, 258, and 260 show exemplary voltages on filter capacitancethat may be observed in embodiments using such “negative” transfers ofcharge. Trace 256 shows the response of an exemplary system whennegligible delay exists when the filter capacitance is reset; such aresponse may be observed in a system that is reset directly withnegligible coupling resistance. Trace 258 shows the response of anexemplary system when a time response similar to a first-order RC or L/Rresponse applies when the filter capacitance is reset; such a responsemay be observed in a system that is reset using an RC or L/R circuit.Trace 260 shows the response of an exemplary system with a time responsethat is roughly linear when the filter capacitance is reset; such aresponse may be observed in a system that is reset by driving a currentsource for an amount of time. Various other techniques for determiningthe measurable capacitance 112 as a function of a representation of acharge on the filter capacitance 110 and the number of times that thecharge transfer process was performed are described herein.

For example, numerous equivalent techniques can be formulated. Withreference to FIG. 3A, an alternate timing scheme 300 illustrates thecircuit shown in FIG. 1B practicing an “oscillator” type method with twodifferent pre-determined voltages to charge measurable capacitance 112and two threshold voltages to determine measurable capacitance 112.Voltage trace 310 of timing scheme 300 applies charging pulses 310 ingroups of opposing “positive” charge pulses 311 and “negative” chargepulses 312. Specifically, timing scheme 300 shows a first set of chargetransfer processes that apply “positive” charge pulses 311 (thusapplying a high pre-determined voltage) during a first time period 301;the timing scheme 300 also shows another set of charge transferprocesses that apply “negative” charge pulses 312 (thus applying a lowpre-determined voltage) during a second time period 302. FIG. 3Aillustrates an exemplary voltage trace 303 of the resulting voltage atnode 113. Voltage trace 303 illustrates the result of the sets ofcharging pulses 311, 312 on the voltage at node 113. Specifically,voltage trace 303 illustrates how each higher charging voltage pulse 311charges measurable capacitance 112 and how the charge thus placed onmeasurable capacitance 112 is discharged when shared with the filtercapacitance 110. Voltage trace 303 additionally illustrates how eachlower charging voltage pulse 312 discharges measurable capacitance 112and how measurable capacitance thus discharges filter capacitance 110when allowed to share with the filter capacitance 110. The resultingvoltage on filter capacitance 110 (i.e. the voltage at node 115 for theembodiment shown by FIGS. 1B, 3A), is illustrated as voltage trace 306.Although timing chart 300 shows the first set of charge transfer processas roughly mirroring that of the other set of charge transfer process,the two charge transfer processes do not need to have such similarity.For example, durations, frequencies, and/or voltage magnitudes candiffer.

In this example, pulses 311 of “positive” charge are applied tomeasurable capacitance 112 during a first period of time 301, such thatallowing the measurable capacitance 112 to share charge with the filtercapacitance 110 charges the filter capacitance 110 at node 115. Thiscauses the voltage on node 115 to rise. At time 315, the voltage at node115 passes a first threshold voltage V_(TH1), which is detected by themeasurement occurring at the time indicated by point 330C. Shortlythereafter, pulses 312 of “negative” charge are applied to measurablecapacitance 112 during the second time period 302, such that allowingthe measurable capacitance 112 to share charge with the filtercapacitance 110 discharges the filter capacitance 110. This causes thevoltage to drop on node 115. At time 317 the voltage at node 117 passesa second threshold voltage V_(TH2), which is detected by the measurementoccurring at the time indicated by point 330E. In the embodiment shownin FIGS. 1B, 3A, both crossings of the threshold voltages V_(TH1) andV_(TH2) are measured and can be used along with the number of times thecharge transfer process was performed to determine the value ofmeasurable capacitance 112.

After periods 301 and 302, additional periods with “positive” chargingand “negative” charging can occur where filter capacitance 110 ischarged during one time period and discharged during another. With itscycles of both “positive” and “negative” charging, this oscillatorembodiment does not need a separate reset signal as described above.This is because the amplitude and rate of change (amount of voltagechange per performance of the charge transfer process) of the voltagewaveform that results on the filter capacitance 110 is independent ofthe starting voltage on the filter capacitance 110 (assuming nocomponents saturate and power supply rails are not reached) and areindicative of the value of the measurable capacitance 112 used in thecharge transfer process. A larger value of measurable capacitance 112means that the voltage thresholds V_(TH1) and V_(TH2) will be crossedwith fewer performances of the charge transfer process, and a smallervalue of measurable capacitance 112 means that the voltage thresholdsV_(TH1) and V_(TH2) will be crossed with more performances of the chargetransfer process. Therefore, it is not required that the filtercapacitance 110 be reset or otherwise placed at a known state after athreshold is crossed and before beginning a set of charge transferprocesses.

Similarly, the cycles of both “positive” and “negative” charging alsomean that additional charge transfer processes can be performed after athreshold voltage is crossed without detrimentally affecting thesensor's performance, even if no reset of the filter capacitance occurs.Taking this into account, sensors with multiple sensing channels drivenin parallel (i.e. by simultaneous charge transfer cycles) wouldpreferably “overrun” sensing channels, and continue performingadditional charge transfer cycles as needed past the crossings of theirrespective thresholds. Sensors with multiple sensing channels inphysical proximity to each other preferably experience similarelectrical states as each other to minimize noise. For example, theeffects of any parasitic coupling between sensing electrodes ofdifferent sensing channels is reduced when the sensing electrodesexhibit similar voltages. If one of these sensing electrodes are drivenby a charge transfer process while another is not, then they can“cross-talk” and degrade sensor performance; however, if both of thesesensing electrodes are driven to similar voltages (say, by similarcharge transfer processes operating simultaneously), then the“cross-talk” is reduced. Driving multiple sensing electrodes to similarvoltages also helps to shield the sensing electrodes from externaldisturbances. Therefore, it is advantageous in such systems to drivethese channels to the same voltage by performing charge transferprocesses on all or none of the channels, no matter what the relativetiming of their threshold crossings.

Therefore, even though the circuits of the present invention can bedriven such that the charge transfer processes end based on when theapplicable threshold voltage (e.g. V_(TH1), V_(TH2) in the case of theembodiment of FIG. 3A) is crossed, that mode of operation may not bedesirable in many embodiments with multiple sensing channels. Instead,it may be preferable to continue performing charge transfer processesafter the crossing of the applicable threshold voltage in a multiplesensing channel system where the channels are operated simultaneously orin parallel. The total number of charge transfer processes performed maythen be based on when the last-in-time filter capacitance of themultiple sensing channels crosses its threshold. The total number ofcharge transfer processes can also be pre-selected to be a large enoughnumber that (at least in most cases) the last-in-time filter capacitanceof the multiple sensing channels would have crossed its threshold.

However, it should be noted that in some embodiments a reset signal maybe applied between sets of opposing charge pulses. For example, thefilter capacitance 110 can be reset-high to produce a high voltage atnode 115 after the voltage at node 115 passes higher threshold V_(TH1),and the filter capacitance 110 can be reset-low to product a low voltageat node 115 after the voltage at node 115 passes lower threshold voltageV_(TH2). Trace 352 of FIG. 3B illustrates the voltage on filtercapacitance 110 for an exemplary system using two reset voltages and twothreshold voltages. Although trace 352 shows no “overrunning” chargetransfer processes past thresholds V_(TH1) or V_(TH2), other embodimentsmay have such “overrunning” processes.

Applying reset signals can also enable variations of the “oscillator”embodiment. For example, when two types of reset signals are used toreset-high and reset-low, an “oscillator” can be implemented using onlyone threshold voltage V_(TH). Trace 354 of FIG. 3B illustrates thevoltage on filter capacitance 110 for an exemplary system using tworeset voltages and one threshold voltage. After a period 355 of chargetransfer processes with “negative” charging pulses 311 sufficient todecrease the voltage on filter capacitance 110 past the thresholdvoltage V_(TH), a reset-low signal can be applied to set the voltage onfilter capacitance 110 to a low-reset voltage lower than V_(TH). Then, aperiod 357 of charge transfer processes with “positive” charging pulses312 can be applied until the voltage on filter capacitance 110 is detectto pass the threshold voltage V_(TH) again, although this time crossingV_(TH) in the opposing manner. Although trace 354 shows no “overrunning”charge transfer processes past thresholds V_(TH1) or V_(TH2), otherembodiments may have such “overrunning” processes.

Returning to the example show by FIGS. 1B, 3A, the number ofperformances of the charge transfer process during time period 301 andthe number of performances of the other charge transfer process duringtime period 302 can be determined in any manner. In various embodiments,the I/O 106 coupled to node 115 incorporates an input having hysteresis,such as Schmitt trigger feature, that provides the twothreshold/comparison voltages V_(TH1) and V_(TH2). I/O 106 can thus beused to read the voltage at node 115 at times indicated by points 330 ofFIG. 3A. As the voltage on filter capacitance 110 is sensed to havepassed higher threshold V_(TH1) (e.g. at point 330C) in suchembodiments, a set of charge transfer process can be applied in theopposing direction to reduce the voltage on filter capacitance 110.Similarly, as the voltage on filter capacitance 110 is sensed to havepassed lower threshold V_(TH2) (e.g. at point 330E) another set ofcharge transfer process can be applied in the opposing direction toincrease the voltage on filter capacitance 110. As shown by trace 306,the sensing scheme 300 produces a voltage at node 115 that approachesthresholds from the correct direction such that hysteretic inputs suchas Schmitt trigger inputs will function correctly in the system andprovide the appropriate thresholds for the periods of charge transferprocesses.

The value of the measurable capacitance can be determined using methodssimilar to the “single-slope” embodiment shown in FIG. 2A. For example,for the embodiment shown in FIG. 3A, the number of charge transferprocess iterations executed to charge the filter capacitance 110 tothreshold voltage V_(TH1) and to discharge the filter capacitance 110 tothreshold voltage V_(TH2) can be identified and used to quantify thevalue of the measurable capacitance 112. For example, the total numberof charging and discharging cycles (i.e. the total number of chargetransfers performed, including both performances of the charge transferprocesses regardless of the charge-up or charge-down effect) can betracked and used to determine the measurable capacitance 112. As anotherexample, the number of charging cycles can be tracked and usedseparately from the number of discharging cycles. Alternatively, onlythe number of charging cycles or only the number of discharging cyclesis tracked and used to determine the measurable capacitance 112; withsuch an embodiment, the type of charge transfer process that is nottracked and used can be used like a reset, and be of much lowerresolution.

Alternatively, and also similar to the “single-slope” embodiment shownin FIG. 2A, the embodiment shown in FIG. 3 can measure therepresentation of the charge on filter capacitance using a componentcapable of providing a multi-bit measurement. For example, themeasurable capacitance 112 can be provided with pre-fixed numbers of“positive” charge pulses 311 and “negative” charge pulses 312, and theresulting voltage at node 115 on filter capacitance 110 can be digitizedinto a multi-bit value one or more times per period (e.g. periods 301,302) to measure the amount of charge transferred without use of separatethresholds V_(TH1) and V_(TH2). These pre-fixed numbers can be fixed bythe system or be quasi-dynamic, and can vary between cycles; forexample, the pre-fixed numbers can be set shortly before beginning orending the cycle of charge transfer processes.

Optionally, the charge on filter capacitance 110 can be “set” to apre-associated value after charging in a first direction using a firsttype of charge transfer process and “reset” (i.e. set to thepre-associated value or another value) after the charging in a seconddirection using a second type of charge transfer process.

In still other embodiments, pre-selected numbers of performances of thecharge transfer process are combined with the use of thresholds. Thatis, the charging and/or discharging processes may execute for apre-determined or pre-established number of cycles, but the chargetransfer process in which the voltage on filter capacitance 110 crossesa threshold voltage is identified. FIG. 3A, for example, shows fourteencharge transfer processes and six measurements 330A-F, with the timethat each measurement 330A-F is taken illustrated by an arrow. In theillustrated example, seven charge transfer processes and threemeasurements are performed during each charging or discharging cycleeven though threshold voltage V_(TH1) is crossed just prior to the thirdsample indicated by point 330C and threshold V_(TH2) is crossed justprior to the second sample indicated by point 330E. A third sample isstill taken, as indicated by point 330F, and a seventh charge transferprocess is still performed even though the second sample indicated thatthe voltage at node 115 had already crossed threshold V_(TH2). Thetaking of such additional samples as 330F is optional. However, suchadditional samples may provide an added advantage, particularly whenmultiple sensing channels are measured using a common filter capacitance110, in that the slope direction is relatively constant across channels,even though the measured capacitance may vary from channel to channel.Such embodiments may also provide other advantages such as in improvedrejection of incorrect readings of a threshold voltage having beencrossed. As discussed above, the performing of additional chargetransfer processes is also optional, but may be especially advantageousin systems having sensing channels in physical proximity to each otherand running these sensing channels in parallel (simultaneously).

It should also be noted that the measurements 330A-F are shown as takenduring later parts of each time period 301, 302 when the possibilitythat the voltage on the filter capacitance 110 will have crossed theapplicable threshold is greater, and not during the earlier part of eachtime period 301, 302 when this possibility is low. With such anapproach, the timing of charging pulses 311 and 312 can be faster andthe pulses more frequent during periods when the voltage on filtercapacitance 110 is not measured. At least the time associated withmeasuring can be removed for such performances.

FIG. 3A shows this additional optional feature in that the pulses 311and 312 used to charge and discharge measurable capacitance 112 need notbe equally spaced in time, and are more frequent earlier in the periods301, 302. As shown in FIG. 3A, the charging and discharging pulsesapplied to measurable capacitance 112 are initially applied fairlyrapidly to speed the sensing process, while later the pulses are appliedmore slowly to ensure complete sharing of the charge on the measurablecapacitance and sufficient time for accurate measurement. In otherembodiments, the measurement period may be faster than thenon-measurement period, as appropriate. This feature and other featuresdescribed earlier are not limited to the bi-directional charging schemeshown in FIG. 3A, and indeed may be incorporated into any of the othercharging techniques described above as well.

Charging and discharging pulses 311, 312 (or any other charging pulse)can also vary in timing for other reasons, and they need not be equallyspaced in time or be of equal duration. In many embodiments, controller102 could process interrupts or other distractions at virtually anypoint of the charge transfer process or determination step, sincevariations in timing are easily tolerated by many of the embodimentsshown herein. This is especially true when the sampling time exceeds thetime constants for settling. Alternately, intentionally varying thespacing in time of pulses may spread the sampling spectrum to yieldimproved immunity to narrowband interference coupled to the measurablecapacitance.

As noted above, many of the embodiments described herein may be readilyimplemented using commercially-available components such as conventionalintegrated circuits and discrete resistors and/or capacitors. Because ofthis simplicity, many of the designs are readily adapted to componentand/or I/O sharing, as described, for example, in FIG. 1C. This sharingconcept may be exploited across many additional sensing channels tocreate sensors capable of efficiently sensing numerous capacitances 112with a single controller 102. This can reduce cost and size of theoverall system significantly. Indeed, various techniques can be formedfor sharing sensing pins on controller 102 and/or any discretecomponents within the passive network across a wide array of alternateembodiments.

In this embodiment shown in FIG. 1C, a capacitive sensor 175 includestwo passive impedances 105A-B. Each passive impedance 105A-B is coupledto a respective measurable capacitance 112A-B and a respective I/O104A-B at a respective node 113A-B. The filter capacitance 110 iscoupled at node 115A to I/O 106A and impedance 105A, and the filtercapacitance 110 is coupled at node I/O 115B to I/O 106B and impedance105B. The capacitance sensor 175 is thus implemented in a manner thatshares filter capacitance 110 between the two measurable capacitances112A-B. In operation, charge transfer processes and determinations canbe performed with the measurable capacitances 112A-B separately. Forexample, when measurable capacitance 112A is active in the chargetransfer process, I/O 106B can be driven to a reference voltage (e.g. apower supply voltage) and measurable capacitance 112A, I/Os 104A and106A, passive impedance 105A, and filter capacitance 110 can functionsimilarly to analogous components shown in FIG. 1B (e.g. measurablecapacitance 112, I/Os 104 and 106, passive impedance 105, and filtercapacitance 110, respectively). Similarly, when measurable capacitance112B is active in the charge transfer process, I/O 106A can be driven toa reference voltage (e.g. a power supply voltage) and measurablecapacitance 112B, I/Os 104B and 106B, passive impedance 105B, and filtercapacitance 110 can function similarly to analogous components shown inFIG. 1B.

By implementing multiple sensing channels on a common controller 102, anumber of efficiencies can be realized. Frequently, sensing electrodesused to provide measurable capacitances 112 can be readily formed on astandard printed circuit board (PCB), so duplication of these elementsis relatively inexpensive in a manufacturing sense. In a case where themeasurable capacitances 112 are expected to be relatively small, thenfilter capacitance 110 may also be manufacturable in a PCB. In addition,none or one or more resistances, capacitances, and inductances may beformed on a PCB to provide impedances used in passive impedance 105 orelsewhere the passive network including passive impedance 105 and filtercapacitance 110. As a result, many of the various features describedherein can be readily implemented using conventional manufacturingtechniques and structures. However, in some cases, components such asfilter capacitance 110, passive impedance 105, and other impedance(s)may be large enough to warrant discrete components in many embodiments.In those cases, these components (e.g. filter capacitance 110) may beimplemented with one or more discrete capacitors, resistors, inductors,and/or other discrete components.

Moreover, the total number of I/O's required and the number ofcomponents in the passive network can be even further reduced throughany sort of time, frequency, code or other multiplexing technique.Arranging the sensing electrodes in various patterns also allows formany diverse types of sensor layouts (including multi-dimensionallayouts used in one, two or more dimensional touchpad sensors) to beformulated. Alternatively, multiple “button”-type touch sensors can bereadily detected using the various sensing channels, or any number ofother sensor layouts could be created.

An alternate embodiment of a capacitance sensor 400 that uses only asingle I/O in controller 102 for each measurable capacitance 112 isshown in FIG. 4A. By using only one I/O for each measurable capacitance,this embodiment allows for an even more efficient implementation of acapacitance sensor in terms of I/O usage, and may be especially usefulfor multi-sensing-channel implementations. Thus, a proximity sensor with20 sensing electrodes could be implemented using 20 I/O's on thecontroller 102. In this embodiment, a single I/O is used to apply thepre-determined voltage to its respective measurable capacitance 112(e.g. I/O 404A is associated with measurable capacitance 112A, and I/O404B is associated with measurable capacitance 112B), to read thevoltage on its respective filter capacitance 110A-B, and also to resetthe charge on its respective filter capacitance 110A-B. The timingdiagram of FIG. 5 shows the operation of a single representative I/O pin404 of FIG. 4A. When multiple pins such as I/O 404A-B are present, thepins may be operated one at a time or simultaneously. FIG. 5 illustratesan operating method analogous to the way that FIG. 2A operates thecircuit of FIG. 1B. Methods analogous to those of FIG. 2B or FIGS. 3A-Bcould also be used to operate the circuit of FIG. 4A (not shown). Asshown in trace 510 of FIG. 5, I/O 404's output includes charging pulses501 that apply the pre-determined voltage to measurable capacitance 112.Between charge pulses 501, I/O 404 is set to a high impedance state suchthat the measurable capacitance 112 is allowed to share charge withfilter capacitance 110 through impedance 105. At reset time 503, I/O 404is set to a low voltage to reset filter capacitor 110. Because filtercapacitor 110 must be reset through passive impedance 105, a relativelylong reset period must be used. Voltage trace 502 illustrates how eachperformance of the charge transfer process including charging voltagepulses 501 and charge sharing between the measurable capacitance 112 andfilter capacitance 110 affects the voltage at node 113. The voltage atnode 113 can be used as the representation of the charge on filtercapacitance 110 and to determine the measured capacitance 112.

It should be noted that in the embodiment illustrated in FIG. 4A thevoltage at nodes 115 cannot be measured directly, as those nodes are notcoupled directly to an I/O of the controller 102. Thus, in thisembodiment the measurable capacitance 112 is determined by measuring theresulting voltage at node 113. Similarly to the alternatives describedabove, in one embodiment the voltage at node 113 can be compared to athreshold voltage V_(TH). This threshold voltage V_(TH) can be selectedto account for the place of measurement at node 113 instead of node 115if applicable, such as when a voltage drop across passive impedance 105can be anticipated and compensated for. Alternatively, the compensationfor an estimable voltage drop can be achieved in calculations performedto determine measurable capacitance 112. Further, as discussed above, insome embodiments and at certain times the voltage at node 113 isessentially equivalent to the voltage at node 115, such that nodifference in measurements exist and needs to be accounted for. Withsuch embodiments, due to the difference in voltage at this node 113compared to the voltage at 115 during at least some portions of thecharge transfer process, it would be desirable to select a time fordetermination where the voltage at node 113 tracks the voltage at node115.

At the end of the charging cycle (e.g. after the voltage at node 113exceeds the threshold voltage V_(TH), as shown in FIG. 5), the I/O 404applies a suitable reset voltage to reset the charge on filtercapacitance 110. In the embodiment of FIGS. 4A and 5, for example, theI/O initially applies pulses of logic “high” values to provide apositive charge to measurable capacitance that is then shared withfilter capacitance 110, and then resets the filter capacitance 110 bydriving a logic “low” or “ground” for a period of time sufficient todischarge filter capacitance 110 through passive impedance 105.Alternatives are contemplated. Again, other sign conventions orequivalent methods of operation could be formulated. As another examplealternative, measurable capacitance 112 may be charged or discharged fora pre-set number of performances of the charge transfer process, withthe voltage on filter capacitance 110 being measured as a multi-bitvalue after the pre-set number of performances rather than as asingle-bit value by comparison with a threshold. In still otherembodiments, bi-directional charge application similar to that shown inFIG. 3A could be applied.

FIG. 4B illustrates another embodiment with capacitance sensor 420. Thisembodiment of FIG. 4B allows further reduction of component count andI/Os used in that impedance 105 and filter capacitance 110 is sharedbetween two sensing channels corresponding to I/O 404A and I/O 404B. Inthis embodiment, the charge transfer process can be performed withmeasurable capacitance 112A by providing charging pulses from I/O 404Ato apply the pre-determined voltage while I/O 404B is set to a referencevalue (e.g. I/O 404B can be set to ground while positive voltage pulsesof +V_(cc) are provided on I/O 404A, or vice versa with I/O 404B set to+V_(cc) while negative voltage pulses of ground are provided on I/O404A. Other convenient voltages such as −V_(cc) or a divided version ofany of these voltages can also be used). Charge can then be allowed toshare by stopping the applying of the pre-determined voltage and holdingI/O 404A to a high impedance state while keeping I/O 404B to the samereference voltage. After one or more performances of this chargetransfer process, the resulting voltage at node 113A and on filtercapacitance 110 can then be read with I/O 404A using any of thetechniques described above. This resulting voltage on filter capacitance110 is representative of the charge on filter capacitance 110, and canbe used with the number of times the charge transfer process wasperformed to produce a value of the measurable capacitance 112. Asimilar charge transfer process can be performed with measurablecapacitance 112B by providing voltage pulses on I/O 404B to apply thepre-determined voltage while holding I/O 404A at a reference value (e.g.ground), and then allowing charge to share between the measurablecapacitance 112B and the filter capacitance 110. One or moremeasurements of the voltage at node 113B can be taken using I/O 404B.These measurement(s) and the number of performances of the chargetransfer process can be used to produce a value of the measurablecapacitance 112B.

FIG. 4C illustrates another capacitance sensor embodiment 440. In thisembodiment, the filter capacitance 110 is shared between two sensingchannels corresponding to I/Os 404A and 404B. Each I/O 404A, 404B iscoupled to a measurable capacitance 112A-B (respectively), and to filtercapacitance 110 via an appropriate respective passive impedance 105A-B.Operation of the sensor would parallel that of sensor 400 shown in FIG.4A, except that the operation of sensors 400 and 440 differ in that eachmeasurable capacitance 112A-B of sensor 440 would be determinedseparately in time without the option of truly contemporaneous chargetransfer processes and measurements. That is, parallel operation wouldtypically be more convenient with the sensor of FIG. 4A than the sensorof FIG. 4C; the FIG. 4C sensor, however, contains one fewer componentwith the sharing of filter capacitance 110. For circuit 440B, I/O 404Bis held at high impedance when measurable capacitance 112A is used in acharge transfer process driven by I/O 404A or when node 113A is beingmeasured using I/O 404A, and I/O 404A held at a high impedance statewhen measurable capacitance 112B and I/O 404B are used in a chargetransfer or when node 113B is being measured using I/O 404B. Constantlycoupling all of the measurable capacitances to the filter capacitanceusing such a scheme may degrade sensor performance (e.g. some charge mayaccumulate onto the inactive measurable capacitances or the inactivemeasurable capacitances may couple in extra noise), such degradation isusually acceptable and often not noticeable. In application, the valueof filter capacitance 110 may be a thousand or more times that of anymeasurable capacitances that the system measures, such that the filtercapacitance will dominate charge accumulation and define the voltage atnode 115. The sensor shown in FIG. 4C could be further modified bycoupling node 115 to an additional I/O on controller 102 (not shown).This additional coupling to the additional I/O can facilitate resettingand measurement of the filter capacitance 110. This additional couplingmay also facilitate sensing methodologies such as those shown in FIG. 3Btraces 352, 354 where fast reset to supply is desirable. The sensorshown in FIG. 4C can also be further modified by coupling the side offilter capacitance 110 opposite node 115 to an additional I/O oncontroller 102 (not shown) instead of ground. This additional couplingto the additional I/O can provide another sensing channel that can beused to measure a measurable capacitance (not shown) coupled to thatnode opposite node 115 without any further additional components. Manyother modifications of sensor 440 exist and are contemplated.

The various concepts and techniques described above can be furtherenhanced in many ways. The exemplary embodiment shown in FIG. 6, forexample, shows a two-channel single-I/O-per-channel implementationsimilar to that of FIG. 4A. In this embodiment, the measurablecapacitances are defined, at least in part, by a plurality of sensorelectrodes 601A-B and an object, such as a finger or stylus (not shown),proximate the sensor electrodes 601A-B. Furthermore, this embodimentincludes a compensation circuit 125 as described above. This embodimentalso includes a guarding electrode 602 that serves to shield the sensorelectrodes 601 from unintended electrical coupling.

In the embodiment shown in FIG. 6, individual sensing electrodes 601A-Bare used to capacitively detect the presence of an object and thusprovide their respective measurable capacitances. During operation, aguarding signal is provided on the guarding electrode 602 using a lowimpedance path 607. The guarding signal helps to shield the sensorelectrodes from unintended coupling with the environment and helps toreduce the net charge transferred from the guarding electrode 602 ontothe filter capacitance(s) (e.g. 110A-B) during the course of the chargetransfer processes leading to the determination of the measurablecapacitance(s). During a portion of the applying of the pre-determinedvoltage, the guard signal can apply a voltage to the guarding electrode602 approximately equal to the voltage applied to the predeterminedvoltage. Then, before the charge sharing between the active sensingelectrode (e.g. 601A-B) with its associated filter capacitance (e.g.110A-B) ends, the voltage of the guard signal may be changed to beapproximately equal to the voltage on the associated filter capacitance(e.g. 110A-B). If a constant voltage is chosen to approximate thevoltage on the associated filter capacitance (an approximation since thevoltage on the filter capacitance changes during and between repetitionsof the charge transfer process), the voltage applied to the guardingelectrode 602 may be changed to a voltage between the appropriatethreshold voltage (V_(TH)) and the voltage on the associated filtercapacitance after reset. The absolute values of the voltages of theguard signal are less important than the voltage swing (i.e. change involtage) of the guard signal. For example, an offset between theguarding electrode voltage and the sensing electrode voltage would notaffect the usefulness of the guard, since for charge transfer through acapacitance, the voltage swing (i.e. change in voltage) is important andthe absolute voltage values are not.

These guarding voltages of the guard signal may be generated in anymanner, such as by tying the guarding electrode 602 to a guardingvoltage generator circuit of any sort. In the exemplary embodiment shownin FIG. 6, an impedance divider circuit 605 suitably produces at leasttwo different values of voltages depending upon the signal applied byI/O 108 of controller 102 and the types and values chosen for thecomponents comprising impedance divider circuit 605. Specifically, animpedance divider 605 composed of a resistive voltage divider can beused. With such a voltage divider, if the signal from I/O 608 is +V_(cc)or if the I/O 608 is held at high impedance, the guarding voltage is+V_(cc). Alternatively, if the signal from I/O 608 is GND, the guardingvoltage is a predetermined fraction of V_(cc) such as (+V_(cc))/2. Asone example, the guarding voltage of the guard signal is changed betweenthe application of the charging pulses that apply the pre-determinedvoltage and the subsequent sharing period. The guarding voltage of theguard signal can also change between repetitions of the charge transferprocess, such as between a reset and the last measurement used in adetermination of the measurable capacitance. Alternate embodiments couldimplement impedance divider circuits with one or more resistances,inductances, or capacitances for ease of design, ease of production,more effective guard signals, and the like. Digital-to-analogconverters, pulse-width modulators, and the like can also be used togenerate the guard signal. The various charge transfer sensingtechniques described herein, coupled with the ease of multi-channelintegration, provide for highly efficient application of guard signals.Nevertheless, guarding is an optional feature that may not be found onall embodiments.

The exemplary sensing circuit 700 of in FIG. 7 shows multiple sensingelectrodes providing multiple measurable capacitances connected tocontroller 102 through a multiplexer 702. In the embodiment shown, twoI/Os 108, 109 of controller 102 are used to provide selection signalsalong paths 706, 708 to multiplexer 702 that select a desired sensorelectrode 601A-D and associated measurable capacitance for sensing.Thus, the multiplexer 702 is used to select which sensor electrode 601is measured for its associated measurable capacitance. Althoughmultiplexer 702 is, strictly speaking, an active device, the presence ofthe multiplexer 702 does not affect the practical operation of thepassive network made up of impedance 105 and filter capacitance 110.That is, whatever sensor electrode 601A-D is selected on multiplexer702, the operation of the sensing circuitry remains otherwise consistentwith the descriptions provided above. Hence, the use of a “passivenetwork” does not entirely preclude the use of one or more activecomponents outside of the passive network and elsewhere in thecapacitance sensor in various alternate embodiments.

With reference now to FIG. 8, an exemplary technique 800 for detecting ameasurable capacitance using switched charge transfer techniquessuitably includes the broad steps of performing a charge transferprocess 801 for two or more times (as repeated by step 806) andsubsequently determining the value of the measurable capacitance (step808). As noted above, the charge transfer process 801 includes applyinga pre-determined voltage to the measurable capacitance (step 802) andthen allowing the measurable capacitance to share charge with a filtercapacitance through a passive impedance that remains coupled to both themeasurable capacitance and to the filter capacitance throughout theapplying (step 802) and allowing steps (step 804). “Sharing” charge inthis context can refer to actively switching that is unrelated tocoupling or decoupling of the passive impedance with the measurablecapacitance or the filter capacitance, otherwise directing the transferof charge, or passively allowing the charge to transfer throughimpedance through quiescence or other inaction. The charge transferprocess can repeat (step 806), and may repeat once, tens, hundreds, ormany more times until the charge on filter capacitance exceeds athreshold voltage, until the process 801 has executed for apre-determined number of times, and/or according to any other scheme.For those embodiments that include the step of resetting the filtercapacitor, this resetting step can be inserted before the firstrepetition of step 802.

With the charge transfer process 801 performed for the appropriatenumber of times, the value of the measurable capacitance is determined(step 808). Although additional charge transfer processes may still beperformed after the appropriate number of performances of the chargetransfer process, these additional charge transfer processes are notused in the determination of the value of the measurable capacitance. Asnoted above, the determination of the measurable capacitance 808 maytake place according to any technique. In various embodiments, thedetermination is made based upon a representation of the amount ofcharge present on the filter capacitance, as well as the number of timesthat the charge transfer process 801 was performed to produce thatrepresentation of the amount of charge. As noted just previously, theparticular number of times that process 801 is performed may bepre-established, determined according to the voltage on filtercapacitance crossing a threshold voltage, or any other factor asappropriate.

Steps 802-808 can be repeated as needed (step 810). For example, in aproximity sensor implementation with multiple sensor electrodes,typically each electrode corresponds to a measurable capacitance. Insuch an implementation, the measurable capacitance corresponding to eachsensor electrode would typically be determined many times per second.This provides the ability to determine the presence of objects near theproximity sensor, and thus facilitates use of the device for user input.Thus, the process can be repeated at a high rate for each sensorelectrode each second.

Process 800 may be executed in any manner. In various embodiments,process 800 is directed by software or firmware residing in a digitalmemory, such as a memory located within or in communication with acontroller, or any other digital storage medium (e.g. optical ormagnetic disk, modulated signal transmitted on a carrier wave, and/orthe like). Process 800 and its various equivalents and derivativesdiscussed above can also be executed with any type or combination ofanalog circuitry, programmed circuitry, or other logic as appropriate.

As stated above, the devices and methods for determining capacitance areparticularly applicable for use in proximity sensor devices. Turning nowto FIG. 9, a block diagram is illustrated of an exemplary electronicsystem 10 that is coupled to a proximity sensor device 11. Electronicsystem 10 is meant to represent any type of personal computer, portablecomputer, workstation, personal digital assistant, video game player,communication device (including wireless phones and messaging devices),media device, including recorders and players (including televisions,cable boxes, music players, and video players) or other device capableof accepting input from a user and of processing information.Accordingly, the various embodiments of system 10 may include any typeof processor, memory or display. Additionally, the elements of system 10may communicate via a bus, network or other wired or wirelessinterconnection. The proximity sensor device 11 can be connected to thesystem 10 through any type of interface or connection, including I2C,SPI, PS/2, Universal Serial Bus (USB), Bluetooth, RF, IRDA, or any othertype of wired or wireless connection to list several non-limitingexamples.

Proximity sensor device 11 includes a controller 19 and a sensing region18. Proximity sensor device 11 is sensitive to the position of an input14 (which can be provided by one or more fingers, styli, and/or otherinput objects) in the sensing region 18, and can detect the input 14 bymeasuring the resulting changes in capacitance due to input 14. “Sensingregion” 18 as used herein is intended to broadly encompass any spaceabove, around, in and/or near the proximity sensor device 11 wherein thesensor is able to detect a position of the object. In a conventionalembodiment, sensing region 18 extends from a surface of the sensor inone or more directions for a distance into space until signal-to-noiseratios prevent input detection. This distance may be on the order ofless than a millimeter, millimeters, centimeters, or more, and may varysignificantly with the sensor design and the sensor performance (e.g.accuracy or resolution) desired. Accordingly, the planarity andcurvature, size, shape and exact locations of the particular sensingregions 18 will vary widely from embodiment to embodiment.

In operation, proximity sensor device 11 suitably detects a position ofinput 14 by measuring the measurable capacitance(s) associated with theplurality of sensing electrodes which are affected by one or morefingers, styli, and/or other objects within sensing region 18. And,using controller 19, proximity sensor device 11 provides electrical orelectronic indicia of the position to the electronic system 10. Thesystem 10 appropriately processes the indicia to accept inputs from theuser for any appropriate purpose and produces any appropriate responses,as discussed earlier.

The proximity sensor device 11 can use discrete electrodes, arrays ofelectrodes, or any other arrangement of capacitive sensor electrodes tosupport any number of sensing regions 18. The proximity sensor devicecan also vary in the type of information provided, such as to provide“one-dimensional” position information (e.g. along a sensing region) asa scalar, “two-dimensional” position information (e.g.horizontal/vertical axes, angular/radial, or any other axes that spanthe two dimensions) as a combination of values, and the like.

The controller 19, sometimes referred to as a proximity sensor processoror touch sensor controller, generally directs the process used tomeasure capacitance using any of the various techniques described above.Here, controller 19 also communicates with the electronic system 10. Thecontroller 19 can perform a variety of additional processes to implementthe proximity sensor device 11. For example, the controller 19 canselect or connect individual measurable capacitances, calculate positionor motion information based on the values of the measurablecapacitances, report a position or motion when a threshold is reached,interpret and wait for a valid tap/stroke/character/button/gesturesequence before reporting it to the electronic system 10 or indicatingit to the user, or any of a multitude of different processes.

In this specification, the term “controller” is defined to include oneor more processing elements that are adapted to perform the recitedoperations. Thus, the controller 19 can comprise all or part of one ormore integrated circuits, firmware code, and/or software code.

Again, as the term is used in this application, the term “electronicsystem” broadly refers to any type of device that communicates withproximity sensor device 11. The electronic system 10 could thus compriseany type of device or devices that a touch sensor device can beimplemented in or coupled to. The proximity sensor device 11 could beimplemented as part of the electronic system 10, or coupled to theelectronic system 10 using any suitable technique. As non-limitingexamples the electronic system 10 could thus comprise any type ofcomputing device, media player, communication device, or another inputdevice (such as another touch sensor device or keypad). In some casesthe electronic system 10 is itself a peripheral to a larger system. Forexample, the electronic system 10 could be a data input or outputdevice, such as a remote control or display device, that communicateswith a computer or media system (e.g., remote control for television)using a suitable wired or wireless technique. It should also be notedthat the various elements (processor, memory, etc.) of the electronicsystem 10 could be implemented as part of an overall system, as part ofthe touch sensor device, or as a combination thereof. Additionally, theelectronic system 10 could be a host or a slave to the proximity sensordevice 11.

It should also be noted that the term “proximity sensor device” isintended to encompass not only conventional proximity sensor devices,but also a broad range of equivalent devices that are capable ofdetecting the position of one or more fingers, pointers, styli and/orother objects. Such devices may include, without limitation, touchscreens, touch pads, touch tablets, biometric authentication devices,handwriting or character recognition devices, and the like. Similarly,the terms “position” or “object position” as used herein are intended tobroadly encompass absolute and relative positional information, and alsoother types of spatial-domain information such as velocity,acceleration, and the like, including measurement of motion in one ormore directions. Various forms of positional information may alsoinclude time history components, as in the case of gesture recognitionand the like. Accordingly, proximity sensor devices can appropriatelydetect more than the mere presence or absence of an object and mayencompass a broad range of equivalents.

It should also be understood that the mechanisms of the presentinvention are capable of being distributed as a program product in avariety of forms. For example, the mechanisms of the present inventioncan be implemented and distributed as a proximity sensor program on acomputer-readable signal bearing media. Additionally, the embodiments ofthe present invention apply equally regardless of the particular type ofsignal bearing media used to carry out the distribution. Examples ofsignal bearing media include: recordable media such as memory cards,optical and magnetic disks, hard drives, and transmission media such asdigital and analog communication links.

Various other modifications and enhancements may be performed on thestructures and techniques set forth herein without departing from theirbasic teachings. Accordingly, there are provided numerous systems,devices and processes for detecting and/or quantifying a measurablecapacitance. While at least one exemplary embodiment has been presentedin the foregoing detailed description, it should be appreciated that avast number of variations exist. The various steps of the techniquesdescribed herein, for example, may be practiced in any temporal order,and are not limited to the order presented and/or claimed herein. Itshould also be appreciated that the exemplary embodiments describedherein are only examples, and are not intended to limit the scope,applicability, or configuration of the invention in any way. Variouschanges can therefore be made in the function and arrangement ofelements without departing from the scope of the invention as set forthin the appended claims and the legal equivalents thereof.

1. A method for measuring a measurable capacitance, the methodcomprising the steps of: performing a charge transfer process for anumber of times equal to at least two, wherein the charge transferprocess comprises the steps of applying a pre-determined voltage to themeasurable capacitance and allowing the measurable capacitance to sharecharge with a filter capacitance through a passive impedance thatremains coupled to both the measurable capacitance and to the filtercapacitance throughout the applying and allowing steps; and determininga value of the measurable capacitance as a function of a representationof a charge on the filter capacitance and the number of times that thecharge transfer process was performed.
 2. The method of claim 1 furthercomprising the step of compensating the representation of the charge onthe filter capacitance for a variation in a reference voltage.
 3. Themethod of claim 2 wherein the compensating step comprises adding atleast a portion of the reference voltage to a voltage on the filtercapacitance.
 4. The method of claim 3 wherein the reference voltage iscoupled to the filter capacitance via an impedance network to add the atleast a portion of the reference voltage to the voltage on the filtercapacitance.
 5. The method of claim 4 wherein the impedance networkcomprises a capacitive divider network.
 6. The method of claim 4 whereinthe impedance network comprises a resistive divider network.
 7. Themethod of claim 1 further comprising the step of resetting the charge onthe filter capacitance.
 8. The method of claim 7 wherein the resettingstep comprises applying a known voltage to the filter capacitance. 9.The method of claim 7 wherein the value of the measurable capacitance isdetermined from the number of times the charge transfer process isperformed between the resetting step and when the representation ofcharge on the filter capacitance is detected to pass a threshold. 10.The method of claim 1 wherein the filter capacitance comprises animpedance divider coupled to two different power supply voltages. 11.The method of claim 10 wherein the impedance divider comprises acapacitive divider.
 12. The method of claim 1 wherein the determiningstep comprises obtaining a multi-bit measurement of a voltage on thefilter capacitance.
 13. The method of claim 1 wherein the determiningstep comprises comparing the representation of charge on the filtercapacitance with a threshold.
 14. The method of claim 1 furthercomprising the step of performing a second charge transfer process for asecond number of times equal to at least one, wherein the second chargetransfer process comprises the steps of applying an opposing voltage tothe measurable capacitance and allowing the measurable capacitance toshare charge with the filter capacitance through the passive impedance,wherein the pre-determined voltage transfers charge in a firstdirection, and the opposing voltage transfers charge in a seconddirection opposite the first direction.
 15. The method of claim 14further comprising the step of setting the representation of the chargeon the filter capacitance to a first value after the performing of thecharge transfer process for the number of times and setting therepresentation of the charge on the filter capacitance to a second valueafter the performing of the second charge transfer process for thesecond number of times.
 16. The method of claim 14 wherein thedetermining step comprises ascertaining one of a number of performancesof the first charge transfer process needed for the representation ofthe charge on the filter capacitance to cross a first threshold and anumber of performances of the second charge transfer processes neededfor the representation of the charge on the filter capacitance to crossa second threshold.
 17. The method of claim 14 wherein the determiningstep comprises: ascertaining a number of performances of the firstcharge transfer process needed for the representation of the charge onthe filter capacitance to cross a first threshold and ascertaining anumber of performances of the second charge transfer processes neededfor the representation of the charge on the filter capacitance to crossa second threshold, and wherein the determining step comprisesascertaining a total number of performances of the charge and secondcharge transfer processes needed for the representation of the charge onthe filter capacitance to cross both a first threshold and a secondthreshold.
 18. The method of claim 14 wherein the charge transferprocess is performed at least until the representation of the charge onthe filter capacitance crosses a first threshold and the second chargetransfer process is performed at least until the representation of thecharge on the filter capacitance crosses a second threshold, and whereinthe first threshold and the second threshold are the same.
 19. Themethod of claim 18 further comprising the steps of setting the charge onthe filter capacitance to a first value after performing the chargetransfer process for the number of times; and setting the charge on thefilter capacitance to a second value different from the first valueafter performing the second charge transfer process for the secondnumber of times.
 20. The method of claim 14 wherein the charge transferprocess is performed at least until the representation of the charge onthe filter capacitance crosses a first threshold and the second chargetransfer process is performed at least until the representation of thecharge on the filter capacitance crosses a second threshold, and whereinthe first threshold differs from the second threshold.
 21. The method ofclaim 20 further comprising the steps of resetting the charge on thefilter capacitance to a reset value after performing the charge transferprocess for the number of times; and resetting the charge on the filtercapacitance to the reset value after performing the second chargetransfer process for the second number of times.
 22. The method of claim1 wherein the determining step comprises measuring a voltage using adigital input threshold of a controller.
 23. The method of claim 22wherein the digital input has hysteresis.
 24. The method of claim 1wherein the determining step comprises measuring a voltage using ananalog-to-digital converter of a controller.
 25. The method of claim 1wherein the charge transfer process further comprises: varying a rate ofexecution of the charge transfer process such that the rate of executionis different during a first portion of the number of times of theperforming of the charge transfer process than during a second portionof the number of times of the performing of the charge transfer process;and comparing the representation of charge on the filter capacitancewith a threshold for the second portion and not for the first portion.26. The method of claim 25 wherein the rate of execution is faster forthe first portion than for the second portion.
 27. The method of claim 1wherein the representation of the charge on the filter capacitance is avoltage on the filter capacitance.
 28. The method of claim 1 wherein theapplying step comprises coupling the measurable capacitance to thepre-determined voltage using a switch.
 29. The method of claim 28wherein the switch comprises a digital output of a controller.
 30. Themethod of claim 28 wherein the switch comprises one of a bufferintegrated circuit and a transistor.
 31. The method of claim 1 whereinthe applying of the predetermined voltage lasts for a period shorterthan a response time of a network comprising the passive impedance. 32.The method of claim 1 further comprising the step of providing a guardsignal configured to guard the measurable capacitance during at least aportion of the charge transfer process.
 33. The method of claim 32wherein a voltage of the guard signal changes during the charge transferprocess.
 34. The method of claim 32 wherein a voltage of the guardsignal changes substantially contemporaneously with at least one of theapplying and allowing steps.
 35. A digital storage medium havingcomputer-executable instructions stored thereon configured to executethe method of claim
 1. 36. A method for measuring a measurablecapacitance, the method comprising the steps of: applying apre-determined voltage to the measurable capacitance; and allowing themeasurable capacitance to share charge through a passive impedance witha filter capacitance, wherein the passive impedance remains coupled toboth the measurable capacitance and the filter capacitance throughoutthe applying and allowing steps; and determining a value of themeasurable capacitance as a function of a voltage on the filtercapacitance.
 37. The method of claim 36 further comprising the step ofadding at least a portion of the reference voltage to the voltage on thefilter capacitance using an impedance coupled to both the referencevoltage to the filter capacitance.
 38. The method of claim 37 whereinthe portion of the reference voltage to the voltage on the filtercapacitance is added using an impedance coupled to both the referencevoltage to the filter capacitance.
 39. A device for measuring ameasurable capacitance, the device comprising: a filter capacitancemeans for storing charge; means for applying a pre-determined voltage tothe measurable capacitance; passive impedance means statically coupledto both the measurable capacitance and the filter capacitance means forallowing the measurable capacitance to share charge with the filtercapacitance means; and means for determining a value of the measurablecapacitance as a function of a representation of a charge on the filtercapacitance means.
 40. A circuit for measuring a measurable capacitanceat a first electrical node, the circuit comprising: a passive impedancestatically coupled between the first electrical node and a secondelectrical node; a filter capacitance coupled between the secondelectrical node and a third electrical node; and a controller coupled tothe first electrical node, wherein the controller is configured toperform a charge transfer process for a number of times greater thanone, wherein the charge transfer process comprises applying apre-determined voltage to the first electrical node for a period of timeand allowing the measurable capacitance to share charge with the filtercapacitance through the passive impedance, and wherein the controller isfurther configured to determine a value of the measurable capacitance asa function of a voltage on the filter capacitance and the number oftimes.
 41. The circuit of claim 40 further comprising a second filtercapacitance coupled between the second electrical node and a fourthelectrical node.
 42. The circuit of claim 41 wherein the thirdelectrical node is coupled to a first power supply voltage, and whereinthe fourth electrical node is coupled to a second power supply voltagedifference from the first power supply voltage.
 43. The circuit of claim40 wherein the third electrical node is connected to a referencevoltage.
 44. The circuit of claim 40 wherein the third electrical nodeis coupled to the controller.
 45. The circuit of claim 44 furthercomprising a second measurable capacitance coupled to the thirdelectrical node.
 46. The circuit of claim 40 wherein the secondelectrical node is coupled to the controller and the third electricalnode is coupled to the controller.
 47. The circuit of claim 46 furthercomprising: a fourth electrical node coupled to the controller; a secondpassive impedance coupled to the third electrical node and to the fourthelectrical node; and a second measurable capacitance coupled to thefourth electrical node.
 48. The circuit of claim 40 further comprisingan impedance coupled to the third electrical node and a referencevoltage, wherein the impedance adds at least a portion of the referencevoltage to the voltage on the filter capacitance.
 49. The circuit ofclaim 40 wherein the passive impedance comprises one of an inductor anda resistor.
 50. The circuit of claim 40 wherein the controller isfurther configured to compare the voltage at the first electrical nodeto a reference voltage to ascertain the voltage on the filtercapacitance.
 51. The circuit of claim 40 wherein the controller isfurther configured to measure the voltage at the first electrical nodeusing a multi-bit analog-to-digital converter to ascertain the voltageon the filter capacitance.
 52. The circuit of claim 51 wherein thecontroller is further configured to compare the voltage at the firstelectrical node with a second reference voltage to ascertain the voltageon the filter capacitance.
 53. The circuit of claim 51 wherein thecontroller is further configured to set the voltage on the filtercapacitance to a known value to reset the filter capacitance.
 54. Thecircuit of claim 53 wherein the first reference voltage and the secondreference voltage are provided by a digital input of the controller, thedigital input having hysteresis.
 55. The circuit of claim 40, whereinthe controller is further connected to the second electrical node. 56.The circuit of claim 55 wherein the controller is further configured tocompare the voltage at the second electrical node with a referencevoltage to ascertain the voltage on the filter capacitance.
 57. Thecircuit of claim 56 wherein the controller is further configured tocompare the voltage at the second electrical node with a secondreference voltage to ascertain the filter capacitance charge,
 58. Thecircuit of claim 57 wherein the first reference voltage and the secondreference voltage are provided by a digital input of the controller, thedigital input having hysteresis
 59. The circuit of claim 40 wherein thefilter capacitance is coupled to a reset voltage through an impedancefor a pre-determined length of time to reset the filter capacitance. 60.A proximity sensor comprising: a sensor electrode having a measurablecapacitance; a switch coupled to the measurable capacitance; a passivenetwork coupled to the measurable capacitance and the switch, thepassive network comprising a passive impedance and a filter capacitance,wherein the passive impedance statically couples the measurablecapacitance to the filter capacitance; and a controller coupled to theswitch, wherein the controller is configured to perform a chargetransfer process for a number of times greater than one, wherein thecharge transfer process comprises applying a pre-determined voltage tothe measurable capacitance using the switch and allowing the measurablecapacitance to share charge with the filter capacitance through thepassive impedance, and wherein the controller is further configured todetermine a value of the measurable capacitance as a function of arepresentation of a charge of the filter capacitance and the number oftimes.
 61. The proximity sensor of claim 60 further comprising: a secondsensor electrode having a second measurable capacitance; and a secondswitch coupled to the second measurable capacitance, a second passivenetwork, wherein the second passive network is coupled to the secondmeasurable capacitance and the second switch, wherein the second passivenetwork further comprises a second passive impedance and a second filtercapacitance, and wherein the second passive impedance statically couplesthe second measurable capacitance to the second filter capacitance; andwherein the controller is coupled to the second switch and is furtherconfigured to perform a second charge transfer process for a secondnumber of times greater than one, wherein the second charge transferprocess comprises applying a second pre-determined voltage to the secondmeasurable capacitance using the second switch and allowing the secondmeasurable capacitance to share charge with the second filtercapacitance through the second passive impedance, and wherein thecontroller is further configured to determine a value of the secondmeasurable capacitance as a function of a representation of a charge ofthe second filter capacitance and the second number of times.
 62. Theproximity sensor of claim 60 further comprising: a second sensorelectrode having a second measurable capacitance; and a second switchcoupled to the second measurable capacitance, wherein the passivenetwork is coupled to the second measurable capacitance and the secondswitch, and wherein the passive impedance statically couples the secondmeasurable capacitance to the filter capacitance, and wherein thecontroller is coupled to the second switch and is further configured toperform a second charge transfer process for a second number of timesgreater than one, wherein the second charge transfer process comprisesapplying a second pre-determined voltage to the second measurablecapacitance using the second switch and allowing the second measurablecapacitance to share charge with the filter capacitance through thepassive impedance, and wherein the controller is further configured todetermine a value of the second measurable capacitance as a function ofa second representation of a second charge of the filter capacitance andthe second number of times.
 63. The proximity sensor of claim 60 furthercomprising: a second sensor electrode having a second measurablecapacitance; which is not connected directly to the passive network. amultiplexer coupled to the first measurable capacitance and the secondmeasurable capacitance and the passive network, wherein the controlleris coupled to the multiplexer and is further configured to multiplex thepassive network at least between the first measurable capacitance andthe second measurable capacitance using the multiplexer, and wherein thecontroller is coupled to the switch and is further configured to performa second charge transfer process for a second number of times greaterthan one, wherein the second charge transfer process comprises applyinga second pre-determined voltage to the second measurable capacitancethrough the multiplexer and allowing the second measurable capacitanceto share charge with the filter capacitance through the passiveimpedance, and wherein the controller is further configured to determinea value of the second measurable capacitance as a function of a secondrepresentation of a second charge on the filter capacitance and thesecond number of.
 64. The proximity sensor of claim 60 wherein themeasurable capacitance represents a capacitance between the sensorelectrode and another electrode.
 65. The proximity sensor of claim 60wherein the measurable capacitance represents a capacitance between thesensor electrode and an external object.
 66. The proximity sensor ofclaim 65 wherein the external object is part of a human body.
 67. Theproximity sensor of claim 60 wherein the controller is furtherconfigured to provide a guard signal configured to guard the measurablecapacitance during at least a portion of the charge transfer process.